A 56-year-old female was referred to our hospital for further examination and treatment because of her increasing right axillary mass for 1 year. Based on histological examination diagnosing the right axillary mass as carcinoma and radiological examination showing no evidence of distal metastasis, we decided to perform a radical resection. The patient underwent right axillary mass resection, axillary lymph node dissection, and latissimus dorsi musculocutaneous flap reconstruction. Right-sided breast cancer was diagnosed based on histopathological examination. The diagnosis was similar to that of breast cancer. The patient underwent adjunctive chemotherapy and is currently undergoing endocrine therapy. The incidence of accessory breast cancer is 0.2-0.6% among all breast cancers and is relatively rare. Postoperative adjuvant pharmacotherapy has no consensus. However, endocrine therapy, chemotherapy, and molecular target therapy are performed in cases of conventional breast cancer. Herein, we describe a case of right axillary accessory breast cancer with skin invasion successfully treated with radical resection.